Hydraulique drive device

ABSTRACT

A hydraulic drive system for a supersized hydraulic working machine such as a hydraulic excavator includes hydraulic pumps connected to main lines through a delivery line and a supply line. Branch portions from the main line include flow control valves for allowing a hydraulic fluid to flow from the hydraulic pumps toward hydraulic cylinders. A hydraulic reservoir is connected to the main lines through a reservoir line and a drain line. Other branch portions for the drain line include flow control valves for allowing a hydraulic fluid to flow from the hydraulic cylinders toward the hydraulic reservoir. A line branched for the delivery line includes a bypass valve for supplying the hydraulic fluid delivered from the hydraulic pumps to the supply line at a desired flow rate and returning the remaining hydraulic fluid to the hydraulic reservoir.

FIELD OF THE INVENTION

The present invention relates to a hydraulic drive system for hydraulicworking machines such as hydraulic excavators, and more particularly toa hydraulic drive system suitable for supersized construction machines.

BACKGROUND ART

A construction of a conventional hydraulic drive system, i.e., oneexample of a hydraulic circuit of the hydraulic drive system whenapplied to, e.g., a supersized hydraulic excavator in excess of 70 t-300t, is shown in FIG. 9 along with a control system thereof.

Specifically, a hydraulic drive system shown in FIG. 9 comprises a firsthydraulic pump 1 a and a second hydraulic pump 1 b both driven by aprime mover 4 a, a third hydraulic pump 3 a and a fourth hydraulic pump3 b both driven by a prime mover 4 b, boom hydraulic cylinders 5 a, 5 band an arm hydraulic cylinder 6 driven by a hydraulic fluid deliveredfrom the first to fourth hydraulic pumps 1 a, 1 b, 3 a, 3 b, a buckethydraulic cylinder 7 driven by the hydraulic fluid delivered from thefirst and third hydraulic pumps 1 a, 3 a, and a swing hydraulic motor 8driven by the hydraulic fluid delivered from the second and fourthhydraulic pumps 1 b, 3 b.

The first hydraulic pump 1 a is connected to the boom hydrauliccylinders 5 a, 5 b, the arm hydraulic cylinder 6 and the buckethydraulic cylinder 7 through a first boom control valve 10 c, a firstarm control valve 10 b, and a first bucket control valve 10 a,respectively. The second hydraulic pump 1 b is connected to the boomhydraulic cylinders 5 a, 5 b, the arm hydraulic cylinder 6 and the swinghydraulic cylinder 8 through a second boom control valve 10 d, a secondarm control valve 10 e, and a first swing control valve 10 f,respectively. These control valves 10 a-10 f constitute a first controlvalve group 10.

The third hydraulic pump 3 a is connected to the boom hydrauliccylinders 5 a, 5 b, the arm hydraulic cylinder 6 and the buckethydraulic cylinder 7 through a third boom control valve 11 c, a thirdarm control valve 11 b, and a second bucket control valve 11 a,respectively. The fourth hydraulic pump 3 b is connected to the boomhydraulic cylinders 5 a, 5 b, the arm hydraulic cylinder 6 and the swinghydraulic cylinder 8 through a fourth boom control valve 11 d, a fourtharm control valve 11 e, and a second swing control valve 11 f,respectively. These control valves 11 a-11 f constitute a second controlvalve group 11.

The bottom sides of the boom hydraulic cylinders 5 a, 5 b are connectedto the first and second boom control valves 10 c, 10 d through mainlines 105 and to the third and fourth boom control valves 11 c, 11 dthrough main lines 125, while the rod sides of the boom hydrauliccylinders 5 a, 5 b are connected to the first and second boom controlvalves 10 c, 10 d through main lines 115 and to the third and fourthboom control valves 11 c, 11 d through main lines 135. The bottom sideof the arm hydraulic cylinder 6 is connected to the first and second armcontrol valves 10 b, 10 e through a main line 116 and to the third andfourth arm control valves 11 b, 11 e through a main line 136, while therod side of the arm hydraulic cylinder 6 is connected to the first andsecond arm control valves 10 b, 10 e through a main line 106 and to thethird and fourth arm control valves 11 b, 11 e through a main line 126.The bottom side of the bucket hydraulic cylinder 7 is connected to thefirst bucket control valve 10 a through a main line 107 and to thesecond bucket control valve 11 a through a main line 127, while the rodside of the bucket hydraulic cylinder 7 is connected to the first bucketcontrol valve 10 a through a main line 117 and to the second bucketcontrol valve 11 a through a main line 137. Further, the swing hydraulicmotor 8 is connected to the first swing control valve 10 f through mainlines 108, 118 and to the second swing control valve 11 f through mainlines 128, 138.

The control system for the hydraulic drive system includes a calculator31 which receives operation signals output from control levers 32, 33and outputs command signals to the front control valves 10 a-f and 11a-f. The control levers 32, 33 are each moved in two orthogonaldirections. Operating the control lever 32 in the two orthogonaldirections outputs a swing operation signal and an arm operation signal,and operating the control lever 33 in the two orthogonal directionsoutputs a boom operation signal and a bucket operation signal.

In the above construction shown in FIG. 9, owing to later-describedrestrictions upon hose diameters available in the market, the main lines105-107, 115-117, 125-127 and 135-137, i.e., high-pressure lines, areeach made up of two or three hoses (or steel pipes, etc.).

DISCLOSURE OF THE INVENTION

The above-explained structure is adapted for a supersized excavator andenables the hydraulic fluid to be supplied at flow rates about twice asmuch by adding the hydraulic pumps 3 a, 3 b, the second control valvegroup 11 and the main lines 125, 126, 127, 128, 135, 136, 137, 138 tothe construction of a conventional large-sized excavator including thehydraulic pumps 1 a, 1 b, the first control valve group 10 and the mainlines 105, 106, 107, 108, 115, 116, 117, 118.

More specifically, a supersized excavator requires the hydraulic fluidto be supplied in a large amount to drive, in particular, the bottomsides of the hydraulic cylinders 5 a, 5 b, 6, 7. Meanwhile, to supplythe hydraulic fluid at a super-high flow rate under a super-highpressure requires that each of the main lines be formed of, e.g., a hoseor a steel pipe having a super-large diameter. In practice, however,since hoses available in the current market have a maximum diameter ofabout 2 inches, the main line must be constructed by arranging aplurality of hoses or the likes (e.g., two or three per main line) sideby side, as mentioned above. This results in that the allowable capacityof the main line is restricted for a supply/return flow rate demanded bythe hydraulic actuator and a relatively large pressure loss is generatedin each of the hoses. Accordingly, in the entire hydraulic circuit ofthe supersized excavator including long lines made of up hoses, steelpipes or the likes, control valves, etc., a very large pressure loss isgenerated and an energy loss is increased correspondingly. Anotherproblem is that the operating speed of the hydraulic actuator is loweredand the working efficiency is reduced.

Further, to arrange a plurality of hoses or the likes to construct onemain line and install two or three main lines on each of the bottom androd sides of the hydraulic cylinders 5 a, 5 b, 6, 7 in the supersizedexcavator is not easy in itself. An additional problem is that thepresence of many hoses or the likes makes poor visibility from a cabtoward the lateral and rear sides of a working machine such as ahydraulic excavator.

An object of the present invention is to provide a hydraulic drivesystem which can reduce the total length of lines made up of hoses,steel pipes or the likes in a supersized hydraulic working machine, andcan lessen a pressure loss in the entirety of a hydraulic circuit.

To achieve the above object, according to the present invention, thereis provided a hydraulic drive system equipped on a hydraulic workingmachine comprising a working machine body and a front device made up ofa plurality of front members coupled to the working machine body to berotatable in the vertical direction, the hydraulic drive systemcomprising a hydraulic reservoir provided on the working machine body,at least one hydraulic pump, a plurality of hydraulic cylinders forrespectively driving the plurality of front members, a plurality of flowcontrol valves provided on the working machine body for respectivelyintroducing a hydraulic fluid delivered from the hydraulic pump to theplurality of hydraulic cylinders and controlling operation of thecorresponding hydraulic cylinders, and a plurality of first connectinglines provided on the front device for respectively connecting the flowcontrol valves and ones of the bottom and rod sides of the correspondinghydraulic cylinders, wherein the hydraulic drive system furthercomprises at least one other hydraulic pump provided on the workingmachine body separately from the aforesaid hydraulic pump, a deliveryline to which is introduced a hydraulic fluid delivered from the otherhydraulic pump and a reservoir line for introducing the hydraulic fluidto the hydraulic reservoir, the delivery line and the reservoir linebeing both provided on the working machine body, a second connectingline provided on the front device and connected at one side thereof tothe delivery line, a plurality of first lines provided on the frontdevice and having one sides connected respectively to the other side ofthe second connecting line so as to be branched therefrom, the othersides of the first lines on the opposite side to the one sides connectedrespectively to at least those of the plurality of first connectinglines which are connected to the bottom sides of the hydrauliccylinders, a plurality of first flow control means provided respectivelyin the plurality of first lines for allowing the hydraulic fluid to flowfrom the other hydraulic pump toward the hydraulic cylinders throughvariable throttles which control respective flows of the hydraulic fluidto desired throttled flow rates, but cutting off flows of the hydraulicfluid from the hydraulic cylinders toward the other hydraulic pump, athird connecting line provided on the front device and connected at oneside thereof to the reservoir line, a plurality of second lines providedon the front device and having one sides connected respectively to theother side of the third connecting line so as to be branched therefrom,the other sides of the second lines on the opposite side to the onesides connected respectively to at least those of the plurality of firstconnecting lines which are connected to the bottom sides of thehydraulic cylinders, a plurality of second flow control means providedrespectively in the plurality of second lines for allowing the hydraulicfluid to flow from the hydraulic cylinders toward the third connectingline through variable throttles which control respective flows of thehydraulic fluid to desired throttled flow rates, but cutting off flowsof the hydraulic fluid from the third connecting line toward thehydraulic cylinders, and third flow control means provided in a linebranched from the delivery line within the working machine body forsupplying the hydraulic fluid delivered from the other hydraulic pump tothe first lines at a desired flow rate and returning the remaininghydraulic fluid to the hydraulic reservoir.

Considering first the extending operation of the hydraulic cylinders,for example, the hydraulic fluid delivered from the at least onehydraulic pump is supplied to those of the first connecting lines, whichare connected to the bottom sides of the hydraulic cylinders, throughthe plurality of control valve switching valves. At this time, thehydraulic fluid delivered from the at least one other hydraulic pump isalso supplied to those of the first connecting lines, which areconnected to the bottom sides of the hydraulic cylinders, through thedelivery line, the second connecting line and the first lines connectedto the second connecting line so as to be branched therefrom at flowrates adjusted by the third flow control means provided in the linebranched from the delivery line and the first flow control meansprovided in the first lines, without passing the flow control valves.This enables the hydraulic fluid to be introduced at a super-high flowrate to the bottom sides of the corresponding hydraulic cylinders in,e.g., a supersized excavator. As a result, the hydraulic cylinders canbe driven in the extending direction to operate the front members.

Considering next the contracting operation of the hydraulic cylinders,for example, part of the return hydraulic fluid from the bottom sides ofthe hydraulic cylinders is introduced to the reservoir line from thoseof the first connecting lines, which are connected to the bottom sidesof the hydraulic cylinders, through the plurality of flow controlvalves. At this time, the remaining return hydraulic fluid from thebottom sides of the hydraulic cylinders is introduced to the reservoirline through the first connecting lines connected to the bottom sides ofthe hydraulic cylinders, the second lines connected to the thirdconnecting line so as to be branched therefrom, and the third connectingline at flow rates adjusted by the second flow control means provided inthe second lines. By thus employing two return routes, the hydrauliccylinders can be driven in the direction to contract for operating thefront members, while draining the return hydraulic fluid at asuper-large flow rate from the bottom sides of the correspondinghydraulic cylinders in, e.g., the supersized excavator.

Here, the conventional structure can also be made adapted for theabove-stated extending and contracting operation of the hydrauliccylinders in a supersized excavator with a super-high flow rate, forexample, by simply adding at least one hydraulic pump, a plurality offlow control valves and a plurality of first connecting lines such thatthe downstream ends of the first connecting lines are connected to thefirst connecting lines which are originally existing. In such a case,however, on the bottom side of each of the hydraulic cylinders, i.e., aboom cylinder, an arm cylinder and a bucket cylinder, provided on thefront device separately in this order from the side of the workingmachine body, there are disposed, e.g., two first connecting lines ashigh-pressure lines respectively led from both a first flow controlvalve group and a second flow control valve group. Accordingly, thenumber of high-pressure lines on the front device from the side of theworking machine body to the bottom sides of the hydraulic cylinders,i.e., the boom cylinder, the arm cylinder and the bucket cylinder, is atotal of six in an area of the front device nearer to the body side thanthe boom cylinder; i.e., two first connecting lines to the bottom sideof the boom cylinder, two first connecting lines to the bottom side ofthe arm cylinder and two first connecting lines to the bottom side ofthe bucket cylinder, is a total of four in an area of the front devicefarther from the body side than the boom cylinder but nearer to the bodyside than the arm cylinder; i.e., two first connecting lines to thebottom side of the arm cylinder and two first connecting lines to thebottom side of the bucket cylinder, and is two in an area of the frontdevice farther from the body side than the arm cylinder but nearer tothe body side than the bucket cylinder; i.e., two first connecting linesto the bottom side of the bucket cylinder.

In the present invention, by contrast, the hydraulic pump, the flowcontrol valves, the other hydraulic pump, the delivery line, thereservoir line and the third flow control means are installed on theworking machine body, whereas the first connecting lines, the secondconnecting line, the third connecting line, the first lines, the secondlines, the first flow control means, the second flow control means andthe hydraulic cylinders are installed on the front device. The number ofhigh-pressure lines led to the bottom sides of the respective hydrauliccylinders, which are particularly problematic from the viewpoint ofpressure loss, is therefore reduced in most areas of the front device ascompared with the case of employing the conventional structure, bylocating the connected positions where the first and second lines arebranched from the second and third connecting lines, respectively, nearthe corresponding hydraulic cylinders such that the first and secondlines are branched to the bottom side of the boom cylinder from thesecond and third connecting lines in positions near the boom cylinder,are branched to the bottom side of the arm cylinder from the second andthird connecting lines in further advanced positions near the armcylinder, and are branched to the bottom side of the bucket cylinderfrom the second and third connecting lines in still further advancedpositions near the bucket cylinder. More specifically, besides the thirdconnecting line as a low-pressure line, the number of high-pressurelines led to the bottom sides of the hydraulic cylinders is reduced intwo areas of the front device as follows. In the area of the frontdevice nearer to the body side than the vicinity of the boom cylinders,there are a total of four lines; i.e., one first connecting line to thebottom side of the boom cylinder, one first connecting line to thebottom side of the arm cylinder, one first connecting line to the bottomside of the bucket cylinder, and one second connecting line. In the areaof the front device farther from the body side than the vicinity of theboom cylinder but nearer to the body side than the vicinity of the armcylinder, there are a total of three lines; i.e., one first connectingline to the bottom side of the arm cylinder, one first connecting lineto the bottom side of the bucket cylinder, and one second connectingline. Since the number of hoses (or steel pipes, etc.) required for allthe high-pressure lines can be thus reduced and the total length of thehigh-pressure lines can be shortened correspondingly, the pressure lossin the entire high-pressure lines can be reduced. In the area of thefront device farther from the body side than the vicinity of the armcylinder but nearer to the body side than the vicinity of the bucketcylinder, there are a total of two lines; i.e., one first connectingline to the bottom side of the bucket cylinder and one second connectingline. Thus, in that area, the number of high-pressure lines required isnot more than but the same as conventional, and therefore the pressureloss is not larger than conventional.

There is also provided a hydraulic drive system preferably modified fromthe above system in that the other side of at least one of the pluralityof first lines on the opposite side to the one side connected to thesecond connecting line is connected to that of the plurality of firstconnecting lines which is connected to the rod side of the hydrauliccylinder, and the first flow control means provided in the at least onefirst line allows the hydraulic fluid to flow from the other hydraulicpump toward the rod side of the hydraulic cylinder through a variablethrottle for controlling a flow of the hydraulic fluid to a desiredthrottled flow rate, but cuts off a flow of the hydraulic fluid from therod side of the hydraulic cylinder toward the other hydraulic pump.

There is further provided a hydraulic drive system preferably modifiedfrom the above system in that the other side of at least one of theplurality of first lines on the opposite side to the one side connectedto the second connecting line is connected to that of the plurality offirst connecting lines which is connected to the rod side of thehydraulic cylinder, the first flow control means provided in the atleast one first line allows the hydraulic fluid to flow from the otherhydraulic pump toward the rod side of the hydraulic cylinder through avariable throttle for controlling a flow of the hydraulic fluid to adesired throttled flow rate, but cuts off a flow of the hydraulic fluidfrom the rod side of the hydraulic cylinder toward the other hydraulicpump, the other side of at least one of the plurality of second lines onthe opposite side to the one side connected to the third connecting lineis connected to that of the plurality of first connecting lines to whichthe at least one first line is connected and which is connected to therod side of the hydraulic cylinder, and the second flow control meansprovided in the at least one second line allows the hydraulic fluid toflow from the rod side of the hydraulic cylinder toward the hydraulicreservoir through a variable throttle for controlling a flow of thehydraulic fluid to a desired throttled flow rate, but cuts off a flow ofthe hydraulic fluid from the hydraulic reservoir toward the rod side ofthe hydraulic cylinder.

Considering first the extending operation of the hydraulic cylinders,for example, the hydraulic fluid delivered from the at least onehydraulic pump is joined with the hydraulic fluid delivered from the atleast one other hydraulic pump, and is then supplied to the bottom sidesof the hydraulic cylinders through the first connecting lines. At thistime, part of the return hydraulic fluid from the rod sides of thehydraulic cylinders is introduced to the reservoir line from those ofthe first connecting lines, which are connected to the rod sides of thehydraulic cylinders, through the plurality of flow control valves, whilethe remaining return hydraulic fluid is introduced to the reservoir linethrough the first connecting lines connected to the rod sides of thehydraulic cylinders, the second lines connected to the third connectingline so as to be branched therefrom, and the third connecting line atflow rates adjusted by the second flow control means provided in thesecond lines.

Considering next the contracting operation of the hydraulic cylinders,for example, the hydraulic fluid delivered from the at least onehydraulic pump is supplied to those of the first connecting lines, whichare connected to the rod sides of the hydraulic cylinders, through theplurality of flow control valves. At this time, the hydraulic fluiddelivered from the at least one other hydraulic pump is also supplied tothose of the first connecting lines, which are connected to the rodsides of the hydraulic cylinders, through the delivery line, the secondconnecting line and the first lines connected to the second connectingline so as to be branched therefrom at flow rates adjusted by the thirdflow control means provided in the line branched from the delivery lineand the first flow control means provided in the first lines, withoutpassing the flow control valves. The return hydraulic fluid from thebottom sides of the corresponding hydraulic cylinders in this case isbranched to one part that is introduced to the plurality of flow controlvalves through of the first connecting lines which are connected to thebottom sides of the hydraulic cylinders, and the other part that isintroduced to the third connecting line through the second lines, boththe parts being finally introduced to the reservoir line.

Here, when the conventional structure is made adapted for theabove-stated extending and contracting operation of the hydrauliccylinders in a supersized excavator with a super-high flow rate, forexample, the number of high-pressure lines to be provided on the frontdevice in its areas from the side of the working machine body to thebottom and rod sides of the hydraulic cylinders a total of twelve in anarea of the front device nearer to the body side than the boom cylinder;i.e., four first connecting lines to the bottom and rod sides of theboom cylinder, four first connecting lines to the bottom and rod sidesof the arm cylinder and four first connecting lines to the bottom androd sides of the bucket cylinder, is a total of eight in an area of thefront device farther from the body side than the boom cylinder butnearer to the body side than the arm cylinder; i.e., four firstconnecting lines to the bottom and rod sides of the arm cylinder andfour first connecting lines to the bottom and rod sides of the bucketcylinder, and is a total of four in an area of the front device fartherfrom the body side than the arm cylinder but nearer to the body sidethan the bucket cylinder; i.e., four first connecting lines to thebottom and rod sides of the bucket cylinder.

In the above construction of the present invention, by contrast, thenumber of high-pressure lines required on both the bottom and rod sidesof the respective hydraulic cylinders can be reduced by locating theconnected positions where the first and second lines are branched fromthe second and third connecting lines, respectively, near thecorresponding hydraulic cylinders. More specifically, in the area of thefront device nearer to the body side than the vicinity of the boomcylinders, there are a total of seven lines; i.e., two first connectinglines to the bottom and rod sides of the boo m cylinder, two firstconnecting lines to the bottom and rod sides of the arm cylinder, twofirst connecting lines to the bottom and rod sides of the bucketcylinder, and one second connecting line. In the area of the frontdevice farther from the body side than the vicinity of the boom cylinderbut nearer to the body side than the vicinity of the arm cylinder, thereare a total of five lines; i.e., two first connecting lines to thebottom and rod sides of the arm cylinder , two first connecting lines tothe bottom and rod sides of the bucket cylinder, and one secondconnecting line. In the area of the front device farther from the bodyside than the vicinity of the arm cylinder but nearer to the body sidethan the vicinity of the bucket cylinder, there are a total of threelines; i.e., two first connecting lines to the bottom and rod sides ofthe bucket cylinder and one second connecting line. As a result, thepressure loss produced in the entire high-pressure lines can be furtherreduced.

There is further provided a hydraulic drive system preferably modifiedfrom the above system in further comprising control means forcontrolling the plurality of flow control valves and the first flowcontrol means to be driven in correlated manners so that just before orafter the hydraulic fluid through at least one of the plurality of flowcontrol valves is sufficiently supplied to the corresponding firstconnecting line, the hydraulic fluid through the corresponding firstflow control means starts to be supplied to the corresponding firstconnecting line.

With this feature, in fine operation where the hydraulic fluid issupplied at a very small flow rate through the flow control valves, nohydraulic fluid is supplied through the first flow control means. Then,at the time or thereabout when the hydraulic fluid is sufficientlysupplied through the flow control valves, the hydraulic fluid is startedto be supplied through the first flow control means. It is thus possibleto suppress a shock that would be otherwise caused upon any actuatorbeing quickly sped-up during the fine operation, and make the operatorfeel less awkward in that occasion.

There is further provided a hydraulic drive system preferably modifiedfrom the above system in further comprising control means for drivingthe first flow control means disposed in at least one of the pluralityof first lines which is connected to the rod side of the hydrauliccylinder, thereby supplying the hydraulic fluid from the other hydraulicpump to the rod side of the hydraulic cylinder, and at the same timedriving the second flow control means disposed in the second line whichis connected to the bottom side of the corresponding hydraulic cylinder,thereby draining the return hydraulic fluid from the bottom side of thecorresponding hydraulic cylinder to the hydraulic reservoir.

There is further provided a hydraulic drive system preferably modifiedfrom the above system in further comprising a plurality of operatingmeans for controlling respective stroke amounts of the plurality of flowcontrol valves and control means for controlling the flow control valvesand the first flow control means to be driven in correlated manners, thecontrol means making control such that in a first input amount areawhere input amounts of the operating means are relatively small, theflow control valves are moved over strokes at a relatively small ratiowith respect to an increase of the input amounts of the operating means,thereby supplying the hydraulic fluid to the corresponding firstconnecting lines, and that in a second input amount area where the inputamounts of the operating means are relatively large, the flow controlvalves are moved over strokes at a relatively large ratio with respectto an increase of the input amounts of the operating means, therebysupplying the hydraulic fluid to the corresponding first connectinglines, and the first flow control means are moved over strokes at apredetermined ratio with respect to an increase of the input amounts ofthe operating means, thereby supplying the hydraulic fluid to thecorresponding first connecting lines through the corresponding firstlines.

Specifically, control at a very small flow rate is performed by movingonly the flow control valves over strokes at a relatively small ratiowith respect to an increase of the input amounts of the operating meansin the first input amount area. After there has reached a flow rateexceeding a certain level, flow rate control is performed through boththe flow control valves and the first flow control means in the secondinput amount area by not only moving the flow control valves overstrokes at a relatively large ratio with respect to an increase of theinput amounts of the operating means, but also moving the first flowcontrol means over strokes at a predetermined ratio. It is thus possibleto suppress a shock that would be otherwise caused upon any actuatorbeing quickly sped-up during the fine operation, and make the operatorfeel less awkward in that occasion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a hydraulic circuit illustrative of theconstruction of a hydraulic drive system according to one embodiment ofthe present invention, along with a control system thereof.

FIG. 2 is a side view showing the entire structure of a hydraulicexcavator which is driven by the hydraulic drive system of FIG. 1.

FIG. 3 is a functional block diagram showing detailed functions of acalculator shown in FIG. 1.

FIG. 4 is a flowchart showing control functions of the calculator shownin FIG. 1.

FIG. 5 is a flowchart showing control functions of the calculator shownin FIG. 1.

FIG. 6 is a graph showing one example of a control lever input amountversus flow rate characteristic.

FIG. 7 is a detailed view showing the construction of a flow controlvalve.

FIG. 8 is a view showing the structure of a seat valve corresponding tothe construction of FIG. 7.

FIG. 9 is a diagram showing a hydraulic circuit illustrative of theconstruction of a conventional hydraulic drive system which is appliedto a supersized hydraulic excavator, along with a control systemthereof.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of a hydraulic drive system according to the presentinvention will be described hereunder with reference to the drawings.

Some embodiments of the present invention will be described withreference to FIGS. 1-8. In these drawings, equivalent members as thosein FIG. 9 showing the conventional structure are denoted by the samereference numbers. This embodiment represents the case where the presentinvention is applied to a supersized hydraulic excavator in excess of 70t-300 t.

First of all, a hydraulic circuit illustrative of the construction ofthe hydraulic drive system according to this embodiment is shown in FIG.1 along with a control system thereof.

Specifically, the hydraulic drive system shown in FIG. 1 comprises afirst hydraulic pump 1 a and a second hydraulic pump 1 b both driven bya prime mover 4 a, a third hydraulic pump 3 a and a fourth hydraulicpump 3 b both driven by a prime mover 4 b, boom hydraulic cylinders 5 a,5 b and an arm hydraulic cylinder 6 driven by a hydraulic fluiddelivered from the first and second hydraulic pumps 1 a, 1 b, a buckethydraulic cylinder 7 driven by the hydraulic fluid delivered from thefirst hydraulic pump 1 a, and a swing hydraulic motor 8 driven by thehydraulic fluid delivered from the second hydraulic pump 1 b.

The first hydraulic pump 1 a is connected to the boom hydrauliccylinders 5 a, 5 b, the arm hydraulic cylinder 6 and the buckethydraulic cylinder 7 through a first boom control valve 10 c, a firstarm control valve 10 b, and a first bucket control valve 10 a,respectively. The second hydraulic pump 1 b is connected to the boomhydraulic cylinders 5 a, 5 b, the arm hydraulic cylinder 6 and the swinghydraulic cylinder 8 through a second boom control valve 10 d, a secondarm control valve 10 e, and a swing control valve 10 f, respectively.These control valves 10 a-10 f constitute a first control valve group10.

The bottom sides of the boom hydraulic cylinders Sa, 5 b are connectedto the first and second boom control valves 10 c, 10 d through a mainline 105 as one first connecting line, while the rod sides of the boomhydraulic cylinders-5 a, 5 b are connected to the first and second boomcontrol valves 10 c, 10 d through a main line 115 as a first connectingline. The bottom side of the arm hydraulic cylinder 6 is connected tothe first and second arm control valves 10 b, 10 e through a main line116 as a first connecting line, while the rod side of the arm hydrauliccylinder 6 is connected to the first and second arm control valves 10 b,10 e through a main line 106 as a first connecting line. The bottom sideof the bucket hydraulic cylinder 7 is connected to the first bucketcontrol valve 10 a through a main line 107 as a first connecting line,while the rod side of the bucket hydraulic cylinder 7 is connected tothe first bucket control valve 10 a through a main line 117 as a firstconnecting line. Further, the swing hydraulic motor 8 is connected tothe swing control valve 10 f through main lines 108, 118 as a firstconnecting lines.

On the other hand, the third and fourth hydraulic pumps 3 a, 3 b areconnected to the main lines 105, 115. 116, 106, 107, 117 through adelivery line 102 to which the hydraulic fluid delivered from thosehydraulic pumps 3 a, 3 b is first introduced, a supply line 100 as asecond connecting line which is provided on a front device 14 (describedlater) of the hydraulic excavator and connected at one side (left sidein the drawing) thereof to the delivery line 102, and respective branchlines 150A, B, C, D, E, F as first lines which are provided on the frontdevice 14 (described later) and connected to the other side of thesupply line 100 in such a manner as being branched from the supply line100 successively. Of those branch lines 150A-F, the branch lines 150A,C, E include first flow control means, e.g., flow control valves 15, 17,19 constructed of solenoid proportional valves with pressurecompensating functions, respectively, which allow the hydraulic fluid toflow from the third and fourth hydraulic pumps 3 a, 3 b toward thebottom sides of the hydraulic cylinders 5 a, 5 b, 6, 7 through variablethrottles for controlling respective flows of the hydraulic fluid todesired throttled flow rates, but cut off reverse flows of the hydraulicfluid, and the branch lines 150B, D, F include first flow control means,e.g., flow control valves 65, 67, 69 constructed of solenoidproportional valves with pressure compensating functions, respectively,which allow the hydraulic fluid to flow from the third and fourthhydraulic pumps 3 a, 3 b toward the rod sides of the hydraulic cylinders5 a, 5 b, 6, 7 through variable throttles for controlling respectiveflows of the hydraulic fluid to desired throttled flow rates, but cutoff reverse flows of the hydraulic fluid.

In this connection, the positions at which the branch lines 150A-F arebranched from the supply line 100 are located near the correspondinghydraulic cylinders (see also FIG. 2 described later). Specifically, thebranch lines 150A, B to the boom cylinders 5 a, 5 b are branched fromthe supply line 100 in positions near the boom cylinders 5 a, 5 b, thebranch lines 150C, D to the arm cylinder 6 are branched from the supplyline 100 in further advanced positions near the arm cylinder 6, and thebranch lines 150E, F to the bucket cylinder 7 are branched from thesupply line 100 in still further advanced positions near the bucketcylinder 7.

A hydraulic reservoir 2 is connected to the main lines 105, 115. 116,106, 107, 117 through a reservoir line 103 for introducing the returnhydraulic fluid to a hydraulic reservoir 2, a drain line 101 as alow-pressure third connecting line which is provided on the front device14 (described later) of the hydraulic excavator and connected at oneside (left side in the drawing) thereof to the reservoir line 103, andrespective branch lines 151A, B, C, D, E, F as second lines which areprovided on the front device 14 (described later) and connected to theother side of the drain line 101 in such a manner as being branched fromthe drain line 101 successively. Of those branch lines 151A-F, thebranch lines 151A, C, E include three second flow control means, e.g.,flow control valves 16, 18, 20 constructed of solenoid proportionalvalves with pressure compensating functions, respectively, which allowthe (return) hydraulic fluid to flow from the bottom sides of thehydraulic cylinders 5 a, 5 b, 6, 7 toward the hydraulic reservoir 2through variable throttles for controlling respective flows of thehydraulic fluid to desired throttled flow rates, but cut off reverseflows of the hydraulic fluid, and the branch lines 151B, D, F includethree second flow control means, e.g., flow control valves 66, 68, 70constructed of solenoid proportional valves, respectively, which allowthe (return) hydraulic fluid to flow from the rod sides of the hydrauliccylinders 5 a, 5 b, 6, 7 toward the hydraulic reservoir 2 throughvariable throttles for controlling respective flows of the hydraulicfluid to desired throttled flow rates, but cut off reverse flows of thehydraulic fluid.

In this connection, the positions at which the branch lines 151A-F arebranched from the drain line 101 are located near the correspondinghydraulic cylinders (see also FIG. 2 described later). Specifically, thebranch lines 151E, F from the bucket cylinder 7 join with the drain line101 in a position near the bucket cylinder 7, the branch lines 151C, Dfrom the arm cylinder 6 join with the drain line 101 in positions nearthe arm cylinder 6 further backing toward a body 13 (described later) ofthe hydraulic excavator, and the branch lines 151A, B from the boomcylinders 5 a, 5 b join with the drain line 101 in positions near theboom cylinders 5 a, Sb still further backing toward the body 13.

Of the above flow control valves 15-20 and 65-70, paris of the flowcontrol valves 15, 16, the flow control valves 17, 18, the flow controlvalves 19, 20, the flow control valves 65, 66, the flow control valves67, 68, and the flow control valves 69, 70 which are disposed inrelatively close relation constitute flow control valve devices 51, 61,71 (see also FIG. 2 described later) and 52, 62, 72.

Further, a line 104 is branched from the delivery line 102 and includesthird flow control means, e.g., a bypass valve 21 constructed of asolenoid proportional valve with a pressure compensating function, forsupplying the hydraulic fluid delivered from the third and fourthhydraulic pumps 3 a, 3 b to the supply line 100 at a desired flow rateand returning the remaining hydraulic fluid to the hydraulic reservoir2. Additionally, between the delivery line 102 and the reservoir line103, there is disposed a relief valve 22 for specifying the maximumpressure in the supply line 100 as a high-pressure line.

The first to fourth hydraulic pumps 1 a, 1 b, 3 a, 3 b, the controlvalve group 10, the delivery line 102, the reservoir line 103, the line104, the bypass valve 21, the relief valve 22, etc. are provided on thebody 13 as shown in FIG. 1, whereas the hydraulic cylinders 5 a, 5 b, 6,7, the supply line 100, the drain line 101, the branch lines 150A-F and151A-F, etc. are provided on the front device 14 as shown in FIG. 1.Also, in the above construction, the third and fourth hydraulic pumps 3a, 3 b each constitute the other hydraulic pump provided on the body 13separately from the first and second hydraulic pumps 1 a, 1 b.

In the above construction shown in FIG. 1, the high-pressure lines,i.e., the main lines 105-107, 115-117, the branch lines 150A-F and thesupply line 100, are each made up of two or three hoses (or steel pipes,etc.). The low-pressure lines, i.e., the branch lines 151A-F and thedrain line 101, may be each formed of one large-diameter hose (or asteel pipe, etc.).

FIG. 2 is a side view showing the entire structure of a hydraulicexcavator which is driven by the hydraulic drive system described above.In FIG. 2, the hydraulic excavator is the backhoe type and comprises thebody 13 as a working machine body, and the front device 14 made up of aplurality of front members, i.e., a boom 75, an arm 76 and a bucket 77,coupled to the body 13 to be rotatable in the vertical direction. Theboom hydraulic cylinder 5, the arm hydraulic cylinder 6 and the buckethydraulic cylinder 7 are mounted respectively on the boom 75, the arm 76and the bucket 77, as shown, and perform the operations of boom-up, armcrowding and bucket crowding when actuated to extend. Also, the swinghydraulic motor 8 shown in FIG. 1 is mounted in a swing base 78 to swingit. Further, though not shown in FIG. 1, travel hydraulic motors fordriving traveling devices 79 of the hydraulic excavator are connected tothe first and second hydraulic pumps 1 a, 1 b through respective controlvalves.

The main lines 105, 115, 106, 116, 107, 117, the supply line 100, thedrain line 101 and the flow control valve devices 51, 61, 71, 52, 62, 72are associated with the front device 14 (but the main line 105 and theflow control valve devices 51, 52, 62, 72 are not shown for the sake ofsimplicity).

Returning to FIG. 1, a calculator 131 is provided as the control systemfor the hydraulic drive system. The calculator 131 receives operationsignals outputed from the control levers 32, 33 and outputs commandsignals to the control valves 10 a-f, the flow control valves 15-20,65-70 and the bypass valve 21. The control levers 32, 33 are each movedin two orthogonal directions. For example, operating the control lever32 in the two orthogonal directions outputs a swing operation signal andan arm operation signal, and operating the control lever 33 in the twoorthogonal directions outputs a boom operation signal and a bucketoperation signal.

FIG. 3 shows a functional block diagram showing detailed functions ofthe calculator 131.

As shown in FIG. 3, the calculator 131 comprises a multiplexer 34 forreceiving the operation signals from the control levers 32, 33 andoutputting any of the operation signals after proper switching andselection, an A/D converter 35 for converting the operation signaloutput from the multiplexer 34 into a digital signal, a RAM 36 fortemporarily storing the A/D converted signal and so on, a ROM 37 forstoring control programs to execute processing procedures describedlater, a central processing unit, i.e., a CPU 38, for processing theoperation signals in accordance with the control programs stored in theROM 37, and output ports 39 for amplifying and outputting outputs of theCPU 38 to the control valves 10 a-f, the flow control valves 15-20,65-70 and the bypass valve 21.

The ROM 37 stores not only general control programs for controlling thecontrol valves 10 a-10 f in accordance with the operation signals fromthe control levers 32, 33, but also control programs for controlling theflow control valves 15-20, 65-70 and the bypass valve 21 followingflowcharts, shown in FIGS. 4 and 5, in accordance with the presentinvention.

The operation of the hydraulic drive system thus constructed will now bedescribed with reference to the flowcharts shown in FIGS. 4 and 5.

In the hydraulic excavator shown in FIG. 2, it is general that when theboom 75, the arm 76 and the bucket 77 constituting the front device 14are operated in the direction to respectively perform the operations ofboom-up, arm crowding and bucket crowding when the hydraulic cylinders 5a, 5 b, 6, 7 are actuated to extend, demanded flow rates are increasedand loads become large. For this reason, the calculator 131 executesprocessing of the operation signals output from the control levers 32,33 for operating the front device 14 in different manners for the armcrowding operation signal, the bucket crowding operation signal and theboom-up operation signal from the other operation signals, i.e., betweenthe operation signals instructing extension of the front hydrauliccylinders 5 a, 5 b, 6, 7 and the other operation signals.

Specifically, when the control levers 32, 33 are first in neutralpositions, the flow control valves 15-20, 65-70 are all closed and thebypass valve 21 is opened, causing the hydraulic fluid from the pumps 3a, 3 b to return to the reservoir 2 through the bypass valve 21. Then,when any of the control levers 32, 33 is operated in the abovecondition, it is determined whether the produced signal from the controllever is one of the boom-up operation signal (abbreviated as theoperation signal (1) hereinafter), the arm crowding operation signal(abbreviated as the operation signal (2) hereinafter), the bucketcrowding operation signal (abbreviated as the operation signal (3)hereinafter), or whether the produced operation signal is one of theboom-down operation signal (abbreviated as the operation signal (4)hereinafter), the arm dumping operation signal (abbreviated as theoperation signal (5) hereinafter) and the bucket dumping operationsignal (abbreviated as the operation signal (6) hereinafter) (step S1).

When the operation signal is one of the operation signals(1)(2)(3)(4)(5)(6), the processing is executed in a different waydepending on which one of the operation signals (1)(2)(3)(4)(5)(6) itis.

More specifically, when the operation signal is (1), the bypass valve 21is closed, the flow control valves 15, 16 are opened, and the other flowcontrol valves 16-20, 65, 67-70 are closed (step S2). Thereby, thehydraulic fluid delivered from the third and fourth hydraulic pumps 3 a,3 b is jointly supplied to the bottom sides of the boom hydrauliccylinders 5 a, 5 b in addition to the hydraulic fluid delivered from thefirst and second hydraulic pumps 1 a, 1 b, and the return hydraulicfluid from the rod sides of the boom hydraulic cylinders 5 a, 5 b isdrained to the hydraulic reservoir 2 through not only the main line 115and the control valves 10 c, 10 d, but also the branch line 151B and thedrain line 101. As a result, the hydraulic cylinders 5 a, 5 b can beoperated to extend at a higher speed or under a higher load.

Likewise, when the operation signal is (2) or (3), the bypass valve 21is closed, the flow control valves 17, 68 or 19, 70 are opened, and theother flow control valves are closed (step S3, S4). Thereby, thehydraulic fluid delivered from the third and fourth hydraulic pumps 3 a,3 b is jointly supplied to the bottom side of the arm hydraulic cylinder6 or the bucket hydraulic cylinder 7, and the return hydraulic fluidfrom the rod side of the arm hydraulic cylinder 6 or the buckethydraulic cylinder 7 is drained to the hydraulic reservoir 2 through notonly the main line 106 or 117 and the control valves 10 b, 10 e or 10 a,but also the branch line 151D or 151F and the drain line 101. As aresult, the hydraulic cylinder 6 or 7 can be operated to extend at ahigher speed or under a higher load.

Further, when the operation signal is (4), the bypass valve 21 isclosed, the corresponding flow control valves 16, 65 are opened, and theother flow control valves are closed (step S5). Thereby, the hydraulicfluid delivered from the third and fourth hydraulic pumps 3 a, 3 b isjointly supplied to the rod sides of the boom hydraulic cylinders 5 a, 5b in addition to the hydraulic fluid delivered from the first and secondhydraulic pumps 1 a, 1 b, and the return hydraulic fluid from the bottomsides of the boom hydraulic cylinders 5 a, 5 b is drained to thehydraulic reservoir 2 through not only the control valves 10 c, 10 d,but also the drain line 101 and the reservoir line 103. As a result, thehydraulic cylinders 5 a, 5 b can be operated to contract at a higherspeed.

Likewise, when the operation signal is (5) or (6), the bypass valve 21is closed, the flow control valves 18, 67 or 20, 69 are opened, and theother flow control valves are closed (step S6, S7). Thereby, thehydraulic fluid delivered from the third and fourth hydraulic pumps 3 a,3 b is jointly supplied to the rod side of the arm hydraulic cylinder 6or the bucket hydraulic cylinder 7, and the return hydraulic fluid fromthe bottom side of the arm hydraulic cylinder 6 or the bucket hydrauliccylinder 7 is drained to the hydraulic reservoir 2 through not only thecontrol valves 10 b, 10 e or 10 a, but also the drain line 101 and thereservoir line 103. As a result, the hydraulic cylinder 6 or 7 can beoperated to contract at a higher speed.

Next, when the operation of the control levers 32, 33 produces two ormore of the operation signals (1)(2)(3)(4) (5)(6), it is determinedwhether those signals are two or not (step S8). If there are two, thenthe processing is executed in a different way depending on which one ofcombinations among the operation signals (1)(2)(3)(4)(5) (6) the twosignals have.

More specifically, when the operation signals are (1)(2), it is firstdetermined whether a difference between input amounts indicated by theoperation signals (1)(2) is not less than a certain value (step S9). Ifthe difference is less than the certain value, then the bypass valve 21is closed, the flow control valves 15, 66 and 17, 68 are shifted underproportional control so that these valves have openings in proportion tothe input amounts of the corresponding operation signals (1)(2), and theother flow control valves are closed (step S10). Thereby, the hydraulicfluid delivered from the third and fourth hydraulic pumps 3 a, 3 b isjointly supplied to the bottom sides of the boom hydraulic cylinders 5a, 5 b and the arm hydraulic cylinder 6 at flow rates distributeddepending on the ratio between the input amounts of the operationsignals (1)(2), and the return hydraulic fluid from the rod sides of theboom hydraulic cylinders 5 a, 5 b and the arm hydraulic cylinder 6 isbranched and drained at flow rates also distributed depending on theratio between the input amounts of the operation signals (1)(2).Accordingly, the combined operation of boom-up and arm crowding can beperformed in a manner adapted for the ratio between the input amountsindicated by the operation signals (1)(2), while utilizing the hydraulicfluid delivered from the third and fourth hydraulic pumps 3 a, 3 b aswell.

If the difference between the input amounts of the operation signals(1)(2) is larger than the certain value and the operation signal (1) islarger than (2), then the bypass valve 21 is closed, the flow controlvalves 15, 66 are opened, and the other flow control valves are closed(step S11). Thereby, the hydraulic fluid delivered from the third andfourth hydraulic pumps 3 a, 3 b is jointly supplied to the bottom sidesof the boom hydraulic cylinders 5 a, 5 b only, and the return hydraulicfluid from the rod sides of the boom hydraulic cylinders 5 a, 5 b onlyis branched and drained to the hydraulic reservoir 2. The reason formaking such control is as follows.

Generally, one of various kinds of work carried out by the hydraulicexcavator is excavating and scooping work in which, after excavatingearth and sand, the bucket 77 is drawn toward the body side to scoop thedug earth and sand in the bucket 77. On this occasion, the bucket 77 isdrawn toward the body side by raising the boom 75 and crowding the arm76. At this time, however, the load pressure for the boom-up operationis extremely large, whereas the load pressure for the arm crowdingoperation is relatively small. To avoid that the hydraulic fluiddelivered from the hydraulic pumps is supplied to only the arm hydrauliccylinder under a light load and the boom-up operation is disabled,therefore, the operator usually manipulates the boom control lever in amaximum input amount and the arm control lever in a very small inputamount. In that combined operation, it is desired to supply thehydraulic fluid to the boom hydraulic cylinders 5 a, 5 b as much aspossible for quickly drawing the bucket 77. Accordingly, if thedifference between the input amounts of the operation signals (1)(2) islarger than the certain value and the operation signal (1) is largerthan (2), then it is judged that the above combined operation is goingto be performed, whereupon the hydraulic fluid delivered from the thirdand fourth hydraulic pumps 3 a, 3 b is supplied to the bottom sides ofthe boom hydraulic cylinders 5 a, 5 b only, as stated above. As aresult, the boom-up operation is quickly performed so that, in theexcavating and scooping work, the bucket is drawn toward the body sidein a shorter time and the working efficiency is improved.

Also, when the operation signals are (1)(3) or (2)(3), the bypass valve21 is closed, the flow control valves 15, 19, 66, 70 or 17, 19, 68, 70are shifted under proportional control so that these valves haveopenings in proportion to the input amounts of the correspondingoperation signals (1)(3) or (2)(3), and the other flow control valvesare closed (step S12 or S13). Thereby, the hydraulic fluid deliveredfrom the third and fourth hydraulic pumps 3 is jointly supplied to thebottom sides of the boom hydraulic cylinders 5 and the bucket hydrauliccylinder 7 or the arm hydraulic cylinder 6 and the bucket hydrauliccylinder 7 at flow rates distributed depending on the ratio between theinput amounts of the operation signals (1)(3) or (2)(3), and the returnhydraulic fluid from the rod sides of the boom hydraulic cylinders 5 andthe bucket hydraulic cylinder 7 or the arm hydraulic cylinder 6 and thebucket hydraulic cylinder 7 is branched and drained at flow rates alsodistributed depending on the ratio between the input amounts of theoperation signals (1)(3) or (2)(3). Accordingly, the combined operationof boom-up and bucket crowding or arm crowding and bucket crowding canbe performed in a manner adapted for the ratio between the input amountsindicated by the operation signals (1)(3) or (2)(3), while utilizing thehydraulic fluid delivered from the third and fourth hydraulic pumps 3 a,3 b as well.

The combined operation instructed by the operation signals (2)(3),particularly, intends to perform excavating by a combination of armcrowding and bucket crowding. It is desired in such excavating work thatthe bucket crowding be surely performed regardless of load fluctuations.With this embodiment, when the load pressure of the bucket hydrauliccylinder 7 is smaller than the load pressure of the arm hydrauliccylinder 6, the hydraulic fluid delivered from the third and fourthhydraulic pumps 3 a, 3 b is also supplied to the bucket hydrauliccylinder 7 in a proportionally distributed manner, enabling theexcavating work to be performed at a higher speed. Further, even whenthe load pressure of the bucket hydraulic cylinder 7 is large, thehydraulic fluid from the third and fourth hydraulic pumps 3 a, 3 b issurely supplied to the bucket hydraulic cylinder 7, and a trouble thatthe bucket hydraulic cylinder 7 would fail to move can be thereforeavoided.

When the operation signals are (1)(5) or (1)(6), the bypass valve 21 isclosed, the flow control valves 15, 18, 66, 67 or 15, 20, 66, 69 areopened, and the other flow control valves are closed (step S14, S15).Thereby, the hydraulic fluid delivered from the third and fourthhydraulic pumps 3 a, 3 b is jointly supplied to the bottom sides of theboom hydraulic cylinders 5 a, 5 b, and the return hydraulic fluid fromthe rod sides of the boom hydraulic cylinders 5 a, 5 b is branched anddrained to the hydraulic reservoir 2. Further, the hydraulic fluiddelivered from the third and fourth hydraulic pumps 3 a, 3 b is jointlysupplied to the rod side of the arm hydraulic cylinder 6 or the buckethydraulic cylinder 7, and the return hydraulic fluid from the bottomside of the arm hydraulic cylinder 6 or the bucket hydraulic cylinder 7is drained to the hydraulic reservoir 2 through not only the controlvalves 10 b, 10 e or 10 a, but also the drain line 101 and the reservoirline 103. Accordingly, the combined operation of boom-up and arm dumpingor bucket dumping can be performed at a high speed with a less pressureloss and high efficiency.

Likewise, when the operation signals are (2)(4) or (2)(6), the bypassvalve 21 is closed, the flow control valves 16, 17, 65, 68 or 17, 20,68, 69 are opened, and the other flow control valves are closed (stepS16, S17). When the operation signals are (3)(4) or (3)(5), the bypassvalve 21 is closed, the flow control valves 16, 19, 65, 70 or 18, 19,67, 70 are opened, and the other flow control valves are closed (stepS18, S19). Thereby, the hydraulic fluid delivered from the third andfourth hydraulic pumps 3 a, 3 b is jointly supplied to the bottom or rodsides of the corresponding hydraulic cylinders, and the return hydraulicfluid from the rod or bottom sides of the hydraulic cylinders is drainedto the hydraulic reservoir 2 through not only the corresponding controlvalves 10, but also the drain line 101 and the reservoir line 103. As aresult, the intended combined operation can be performed at a high speedwith a less pressure loss and high efficiency.

Also, when the operation signals are (4)(5) or (4)(6), the bypass valve21 is closed, the flow control valves 16, 18, 65, 67 or 16, 20, 65, 69are shifted under proportional control so that these valves haveopenings in proportion to the input amounts of the correspondingoperation signals (4)(5) or (4)(6), and the other flow control valvesare closed (step S20, S21). Thereby, the hydraulic fluid delivered fromthe third and fourth hydraulic pumps 3 a, 3 b is jointly supplied to therod sides of the boom hydraulic cylinders 5 a, 5 b and the arm hydrauliccylinder 6 or the bucket hydraulic cylinder 7 at flow rates distributeddepending on the ratio between the input amounts of the operationsignals (4)(5) or (4)(6). Further, the return hydraulic fluid from thebottom sides of the boom hydraulic cylinders 5 a, 5 b and the armhydraulic cylinder 6 and the bucket hydraulic cylinder 7 is drained tothe hydraulic reservoir 2 through not only the control valves 10 c, 10 dand 10 b, 10 e or 10 a, but also the drain line 101 and the reservoirline 103 at flow rates also distributed depending on the ratio betweenthe input amounts of the operation signals (4)(5) or (4)(6).Accordingly, the combined operation of boom-down and arm dumping orbucket dumping can be performed at a higher speed with a less pressureloss and high efficiency.

Likewise, when the operation signals are (5)(6), the bypass valve 21 isclosed, the flow control valves 18, 20, 67, 69 are shifted underproportional control so that these valves have openings in proportion tothe input amounts of the corresponding operation signals (5)(6), and theother flow control valves are closed (step S22). Thereby, the hydraulicfluid delivered from the third and fourth hydraulic pumps 3 a, 3 b isjointly supplied to the rod sides of the arm hydraulic cylinder 6 andthe bucket hydraulic cylinder 7 at flow rates distributed depending onthe ratio between the input amounts of the operation signals (5)(6).Further, the return hydraulic fluid from the bottom sides of the armhydraulic cylinder 6 and the bucket hydraulic cylinder 7 is drained tothe hydraulic reservoir 2 through not only the control valves 10 b, 10 eand 10 a, but also the drain line 101 and the reservoir line 103 at flowrates also distributed depending on the ratio between the input amountsof (5)(6). Accordingly, the combined operation of arm dumping and bucketdumping can be performed at a higher speed with a less pressure loss andhigh efficiency.

When the operation of the control levers 32, 33 produces three of theoperation signals (1)(2)(3)(4) (5)(6), the processing is executed in adifferent way depending on which one of combinations among the operationsignals (1)(2)(3)(4)(5)(6) the three signals have.

More specifically, when the operation signals are (1)(2)(3), the bypassvalve 21 is closed, the flow control valves 15, 66 are opened, and theother flow control valves are closed (step S23).

The combined operation instructed by the operation signals (1)(2)(3)includes horizontal drawing work in which the ground surface afterexcavating is leveled by crowding both the arm 76 and the bucket 77while raising the boom 75. In such horizontal drawing work, the loadpressures of the boom hydraulic cylinders 5 a, 5 b are much larger thanthe load pressures of the arm and bucket hydraulic cylinders 6, 7. Forthis reason, the hydraulic fluid delivered from the third and fourthhydraulic pumps 3 a, 3 b is exclusively supplied to the bottom sides ofthe boom hydraulic cylinders 5 a, 5 b, as mentioned above, so that thehydraulic fluid can be surely supplied to the boom hydraulic cylinders 5a, 5 b subjected to a large load and the horizontal drawing work can besmoothly performed.

Also, when the operation signals are (1)(2)(6), the bypass valve 21 isclosed, the flow control valves 15, 17, 20, 66, 68, 69 are opened, andthe other flow control valves are closed (step S24). Thereby, thehydraulic fluid delivered from the third and fourth hydraulic pumps 3 a,3 b is jointly supplied to the bottom sides of the boom hydrauliccylinders 5 a, 5 b and the arm hydraulic cylinder 6, and the returnhydraulic fluid from the rod sides of the boom hydraulic cylinders 5 a,5 b and the arm hydraulic cylinder 6 is branched and drained to thehydraulic reservoir 2 through the main lines 115, 106 and through thebranch lines 151B, 151D and the drain line 101. Further, the hydraulicfluid delivered from the third and fourth hydraulic pumps 3 a, 3 b isjointly supplied to the rod side of the bucket hydraulic cylinder 7, andthe return hydraulic fluid from the bottom side of the bucket hydrauliccylinder 7 is drained to the hydraulic reservoir 2 through not only thecontrol valve 10 a, but also the drain line 101 and the reservoir line103. Accordingly, the combined operation of boom-up, arm crowding andbucket dumping can be performed at a high speed with a less pressureloss and high efficiency.

Likewise, when the operation signals are (1)(3)(5), the bypass valve 21is closed, the flow control valves 15, 18, 19, 66, 67, 70 are opened,and the other flow control valves are closed (step S25). When theoperation signals are (1)(5)(6), the bypass valve 21 is closed, the flowcontrol valves 15, 18, 20, 66, 67, 69 are opened, and the other flowcontrol valves are closed (step S26). When the operation signals are(2)(3)(4), the bypass valve 21 is closed, the flow control valves 16,17, 19, 65, 68, 70 are opened, and the other flow control valves areclosed (step S27). When the operation signals are (2)(4)(6),.the bypassvalve 21 is closed, the flow control valves 16, 17, 20, 65, 68, 69 areopened, and the other flow control valves are closed (step S28). Whenthe operation signals are (3)(4)(5), the bypass valve 21 is closed, theflow control valves 16, 18, 19, 65, 67, 70 are opened, and the otherflow control valves are closed (step S29). When the operation signalsare (4)(5)(6), the bypass valve 21 is closed, the flow control valves16, 18, 20, 65, 67, 69 are opened, and the other flow control valves areclosed (step S30).

Thus, the hydraulic fluid is supplied to the bottom (or rod) sides ofthe corresponding hydraulic cylinders through not only the controlvalves, but also the supply line 100 and corresponding ones of thebranch lines 150A-E. Also, the return hydraulic fluid from the rod (orbottom) sides of the corresponding hydraulic cylinders is drained to thehydraulic reservoir 2 through not only the control valves, but also thedrain line 101 and the reservoir line 103. Consequently, the combinedoperation intended by the operator can be performed at a high speed witha less pressure loss and high efficiency.

In the process of carrying out the various combined operations statedabove, the calculator 131 performs a function of control means forcontrolling the control valves 10 a-f and the flow control valves 15,17, 19, 65, 67, 69 to be driven in correlated manners explained below inaccordance with the general control programs which are stored in the ROM37 (see FIG. 3) and control the control valves 10 a-10 f in response tothe operation signals from the control levers 32, 33. FIG. 6 shows oneexample of details of control executed by the calculator 131, andrepresents flow rate characteristics (solid lines) of the control valves10 a-f and flow rate characteristics (broken lines (1) or (2)) of theflow control valves 15, 17, 19, 65, 67, 69 with respect to the controllever input amount. As seen from FIG. 6, first, in an area (first inputamount area) where the input amounts of the control levers 32, 33 arerelatively small, only the control valves 10 a-f are moved over strokesat a relatively small ratio with respect to an increase of the inputamount, thereby supplying the hydraulic fluid to the corresponding mainlines 105-107, 115-117. Then, in an area (second input amount area)where the input amounts of the control levers 32, 33 are relativelylarge, i.e., after a position at which the flow rate through any of thecontrol valves 10 a-f starts to rise quickly with an increase of thelever input amount, the control valves 10 a-f are moved over strokes ata relatively large ratio with respect to an increase of the inputamount, thereby supplying the hydraulic fluid to the corresponding mainlines 105-107, 115-117. At this time, the flow control valves 15, 17,19, 65, 67, 69 are also moved over strokes substantially at the sameratio as for the control valves 10 a-f with respect to an increase ofthe input amount. On the characteristic curves of control lever inputamount versus flow rate shown in FIG. 6, positions (input amounts x1,x2) at which the flow control valves 15, 17, 19, 65, 67, 69 start tosupply the hydraulic fluid correspond to a position xo at which thecharacteristic curve of the control valves 10 a-f starts to rise quickly(including the vicinity of the rising-start position). Upon the movementof the flow control valves, the hydraulic fluid is supplied to thecorresponding main lines 105-107, 115-117 through the correspondingbranch lines 150A-F. Accordingly, just before or after the hydraulicfluid through the control valves 10 a-f is sufficiently supplied to thecorresponding main lines 105, 116, 107 or 115, 106, 117, the hydraulicfluid through the corresponding flow control valves 15, 17, 19 or 65,57, 69 starts to be supplied to the main lines 105, 116, 107 or 115,106, 117 from the branch lines 150A, C, E or 150B, D, F. As a result, atthe time the flow control valves 15, 17, 19 or 65, 57, 69 are switchedover, it is possible to prevent the actuators from speeding up soabruptly as to cause shocks, or make the operator feel less awkward inoperation.

In this embodiment, as explained above, the various combined operationscan be performed at a high speed with a less pressure loss and highefficiency by controlling the flow control valves 15-20, 65-70 and thebypass valve 21 to be selectively opened and closed. Additionally, thegreatest feature of this embodiment is to reduce the total length of thelines, such as hoses or steel pipes, in a supersized excavator, and tolessen the entire pressure loss of a hydraulic circuit thereof. Thismain advantage will be described below in detail.

In the hydraulic drive system of this embodiment, when the hydrauliccylinders are operated in the direction to extend, the hydraulic fluiddelivered from the hydraulic pumps 1 a, 1 b is supplied to thecorresponding main lines 105, 116, 107 through the control valve group10. At this time, the hydraulic fluid delivered from the hydraulic pumps3 a, 3 b is also supplied to the main lines 105, 116, 107 through thedelivery line 102, the supply line 100 and the branch lines 150A, C, Eat flow rates adjusted by the bypass valve 21 and the flow controlvalves 15, 17, 19 in the branch lines 150A, C, E, without passing thecontrol valve group 10. The hydraulic fluid supplied to the main lines105, 116, 107 is then introduced to the bottom sides of thecorresponding hydraulic cylinders 5 a, 5 b, 6, 7 to drive them, therebyoperating the front members 75, 76, 77. On the other hand, the returnhydraulic fluid from the rod sides of the hydraulic cylinders 5 a, 5 b,6, 7 is simultaneously drained to the hydraulic reservoir 2 from themain lines 115, 106, 117 through the control valve group 10, and inaddition also drained to the hydraulic reservoir 2 through the branchlines 151B, D, F and the drain line 101 at flow rates adjusted by theflow control valves 66, 68, 70 in the branch lines 151B, D, F, withoutpassing the control valve group 10.

Next, when the hydraulic cylinders are operated in the direction tocontract, for example, the hydraulic fluid delivered from the hydraulicpumps 1 a, 1 b is supplied to the corresponding main lines 115, 106, 117through the control valve group 10. At this time, the hydraulic fluiddelivered from the hydraulic pumps 3 a, 3 b is also supplied to the mainlines 115, 106, 117 through the delivery line 102, the supply line 100and the branch lines 150B, D, F at flow rates adjusted by the bypassvalve 21 and the flow control valves 65, 67, 69 in the branch lines150B, D, F, without passing the control valve group 10. The hydraulicfluid supplied to the main lines 115, 106, 117 is then introduced to therod sides of the corresponding hydraulic cylinders 5 a, 5 b, 6, 7 todrive them, thereby operating the front members 75, 76, 77. On the otherhand, part of the return hydraulic fluid from the bottom sides of thehydraulic cylinders 5 a, 5 b, 6, 7 is simultaneously drained to thehydraulic reservoir 2 from the main lines 105, 116, 107 through thecontrol valve group 10. In addition, the remaining return hydraulicfluid is drained to the hydraulic reservoir 2 through the main lines105, 116, 107, the branch lines 151A, C, E, the drain line 101 and thereservoir line 103 at flow rates adjusted by the flow control valves 16,18, 20 disposed in the branch lines 151A, C, E. By thus employing tworeturn routes, the hydraulic cylinders 5 a, 5 b, 6, 7 can be driven inthe direction to contract for operating the front members 75, 76, 77,while draining the return hydraulic fluid at a super-large flow ratefrom the bottom sides of the hydraulic cylinders 5 a, 5 b, 6, 7.

Here, the conventional structure can also be employed as a measure forrealizing a super-high flow rate in the supersized excavator intended bythis embodiment. In other words, the super-high flow rate can berealized by simply adding the hydraulic pumps 3 a, 3 b, the controlvalve group 11 and the main lines 125-127, 135-137 such that thedownstream ends of the main lines 125-127, 135-137 are connected to theoriginally existing main lines 105-107, 115-117, as shown FIG. 9 before.In such a case, however, a large number of high-pressure lines wouldhave to be routed along the front device 14 from the body side to therespective cylinders. Specifically, in an area (conceptually indicatedby D in FIG. 9) of the front device 14 nearer to the body side than theboom cylinders 5 a, 5 b, there are routed a total of twelve lines; i.e.,the four main lines 105, 125, 115, 135 to the bottom and rod sides ofthe boom cylinders 5 a, 5 b, the four main lines 116, 136, 106, 126 tothe bottom and rod sides of the arm cylinder 6, and the four main lines107, 127, 117, 137 to the bottom and rod sides of the bucket cylinder 7.In an area (conceptually indicated by E in FIG. 9) of the front device14 farther from the body side than the boom cylinders 5 a, 5 b butnearer to the body side than the arm cylinder 6, there are routed atotal of eight lines; i.e., the four main lines 116, 136, 106, 126 tothe bottom and rod sides of the arm cylinder 6 and the four main lines107, 127, 117, 137 to the bottom and rod sides of the bucket cylinder 7.In an area (conceptually indicated by F in FIG. 9) of the front device14 farther from the body side than the arm cylinder 6 but nearer to thebody side than the bucket cylinder 7, there are routed the four mainlines 107, 127, 117, 137 to the bottom and rod sides of the bucketcylinder 7.

In the hydraulic drive system of this embodiment, of the presentinvention by contrast, the hydraulic pumps 1 a, 1 b and 3 a, 3 b, thecontrol valves 10 a-f, the delivery line 102, the reservoir line 103 andthe bypass valve 21 are installed on the body 13 of the hydraulicexcavator, whereas the main lines 105, 115, 116, 106, 107, 117, thesupply line 100, the drain line 101, the branch lines 150A-F and 151A-F,the flow control valves 15-20 and 65-70, and the hydraulic cylinders 5a, 5 b, 6, 7 are installed on the front device 14. In addition, thepositions where the branch lines 150A-F or 151A-F are branched from thesupply line 100 or the drain line 101 are located near the correspondinghydraulic cylinders. The number of high-pressure lines led to the bottomand rod sides of the respective hydraulic cylinders, which areparticularly problematic from the viewpoint of pressure loss, istherefore reduced in most areas of the front device 14 as compared withthe system of FIG. 9 employing the conventional structure.

To explain it in more detail, besides the drain line 101 as alow-pressure line, the number of high-pressure lines is reduced asfollows. In an area (conceptually indicated by A in FIG. 1) of the frontdevice 14 nearer to the body side than the vicinity of the boomcylinders 5 a, 5 b, a total of only seven lines are required to berouted; i.e., the two main lines 105, 115 to the bottom and rod sides ofthe boom cylinders 5 a, 5 b, the two main lines 116, 106 to the bottomand rod sides of the arm cylinder 6, the two main lines 107, 117 to thebottom and rod sides of the bucket cylinder 7, and the one supply line100. In an area (conceptually indicated by B in FIG. 1) of the frontdevice 14 farther from the body side than the vicinity of the boomcylinders 5 a, 5 b but nearer to the body side than the vicinity of thearm cylinder 6, a total of only five lines are required to be routedi.e., the two main lines 116, 106 to the bottom and rod sides of the armcylinder 6, the two main lines 107, 117 to the bottom and rod sides ofthe bucket cylinder 7, and the one supply line 100. In an area(conceptually indicated by C in FIG. 1) of the front device 14 fartherfrom the body side than the vicinity of the arm cylinder 6 but nearer tothe body side than the vicinity of the bucket cylinder 7, a total ofonly three lines are required to be routed; i.e., the two main lines107, 117 to the bottom and rod sides of the bucket cylinder 7 and theone supply line 100.

Thus, in the areas indicated by D, E, F in FIG. 9 and A, B, C in FIG. 1,the hydraulic drive system of this embodiment of the present inventioncan reduce the number of high-pressure lines on each of the bottom androd sides as compared with the case of employing the conventionalstructure. The total length of hoses, steel pipes or the likesconstituting the high-pressure lines can therefore be shortened.

As explained above, this embodiment can reduce the number ofhigh-pressure lines as compared with the case of employing theconventional structure, and the total length of hoses, steel pipes orthe likes can be shortened correspondingly as a whole of the hydraulicexcavator. Accordingly, a pressure loss in the entirety of the hydrauliccircuit can be reduced, thus making it possible to lessen the energyloss, increase the operating speeds of the hydraulic cylinders, andimprove the working efficiency. Further, by increasing the diameter of ahose, a steel pipe or the like as far as possible which constitutes thedrain line 101 as a low-pressure line, the pressure loss can be furtherreduced.

Comparing FIG. 9 employing the conventional structure and FIG. 1 of thisembodiment from the standpoint of valves, the control valves 11 a-f inFIG. 9 are replaced by the flow control valves 15-20, 65-70 and thebypass valve 21. The flow control valves 15-20, 65-70 and the bypassvalve 21 which are individual valves are generally easier to be adaptedfor an increase in capacity than the control valves 11 in FIG. 9. Thisalso contributes to reducing the pressure loss remarkably.

Also, with this embodiment, when the control levers 32, 33 are in theneutral positions, the flow control valves 15-20, 65-70 are all closedand the bypass valve 21 is opened, causing the hydraulic fluid from thepumps 3 a, 3 b to return to the reservoir 2 through the bypass valve 21.Accordingly, the bypass valve 21 is disposed midway of the shortestdistance between the pumps 3 a, 3 b and the hydraulic reservoir 2. Thisprovides another advantage that the loss caused in the neutral conditionof the control levers 32, 33 can be minimized to a lower level thancaused in the case of FIG. 9 employing the conventional structure.

While the above embodiment includes the branch lines 150B, D, F and151B, D, F having one sides connected to the main lines 115, 106, 117which are in turn connected to the rod sides of the hydraulic cylinders5 a, 5 b, 6, 7, and the flow control valves 65, 66, 67, 68, 69, 70provided respectively in those branch lines, the above branch lines andflow control valves are not necessarily provided. In general, because ahydraulic cylinder has a capacity difference of about twice between thebottom side and the rod side, the rod side does not often require aslarge a flow rate as required on the bottom side even in a supersizedexcavator in which a super-high flow rate is to be achieved. In such acase, the hydraulic circuit on the rod side may be arranged such thatthe hydraulic fluid is supplied and returned trough the control valvegroup 10 as per conventional. Alternatively, the hydraulic fluid fromthe third and fourth hydraulic pumps may be joined with the rod sides ofonly desired ones of the hydraulic cylinders. Further, only the branchlines 151B, 151D, 151F and the flow control valves 66, 68, 70corresponding to those branch lines may be disposed on the rod sides ofthe hydraulic cylinders so that when the hydraulic cylinders areoperated to extend, the return hydraulic fluid from the rod sides arereturned to the reservoir through the control valves 10 and the drainline 101 for reducing the pressure loss of the return hydraulic fluid.Other various combinations are also conceivable.

In the above embodiment, the hydraulic fluid for the swing hydraulicmotor 8 is supplied and returned through the control valve 10 f as perconventional, but the present invention is not limited to such anarrangement. As with the other hydraulic cylinders 5 a, 5 b, 6, 7, thehydraulic fluid to the swing hydraulic motor 8 may also be jointlysupplied through the supply line 100, and/or the return hydraulic fluidtherefrom may also be jointly drained through the drain line 101. Thismodified case can also provide the similar advantages as mentionedabove.

While the above embodiment is designed to shift the flow control valvesunder proportional control depending on the input amounts only in stepsS10, S12, S13, S20, S21, S22 in FIG. 4 when there are two or threeoperation signals, the present invention is not limited to such anarrangement. It is apparent that, in any of the other combinedoperations (steps S11, S14-S19, S23-S30), the flow control valves mayalso be shifted under proportional control depending on the kinds ofwork and so on, if desired, without departing from the gist of thepresent invention. On the contrary, in any of steps S10, S12, S13, S20,S21, S22 which have been explained above as performing proportionalcontrol, the flow control valves may also be shifted under notproportional control, but normal on/off control if the proportionalcontrol is not particularly required in consideration of the kinds ofwork and so on.

While the above embodiment is designed to determine a difference betweenthe input amounts in S9 for only the combination of the signals (1)(2)and to perform different control manners between S10 and S11 dependingon the difference when there are two or three operation signals in FIG.4, the present invention is not limited to such an arrangement. Forexample, the processing may also be executed for the combination of thesignals (1)(5) (step S14) by determining a difference of the inputamounts and opening only the flow control valves 15, 66 for the boomhydraulic cylinders 5 a, 5 b when the difference is not less than acertain value. In this case, the following meaning is resulted.

Generally, one of various kinds of work carried out by a hydraulicexcavator is dump loading work for loading dug earth and sand on a dumptruck. In such work, the arm 76 is dumped while swinging the swing baseand raising the boom 75. At this time, the load pressure for the boom-upoperation is extremely large, whereas the load pressure for the armdumping operation is relatively small. To avoid that the hydraulic fluiddelivered from the hydraulic pumps is supplied to only the arm hydrauliccylinder under a light load and the boom-up operation is disabled,therefore, the operator usually manipulates the boom control lever in amaximum input amount and the arm control lever in a very small inputamount. In that combined operation, it is desired to supply thehydraulic fluid to the boom hydraulic cylinders 5 a, 5 b as much aspossible for quickly raising the bucket 77. Accordingly, as with stepS9, if the difference between the input amounts of the operation signals(1)(5) is larger than the certain value and the operation signal (1) islarger than (5), then it is judged that the above combined operation isgoing to be performed, whereupon the hydraulic fluid delivered from thethird and fourth hydraulic pumps 3 a, 3 b is supplied to the bottomsides of the boom hydraulic cylinders 5 a, 5 b only. As a result, theboom-up operation is quickly performed so that, in the dump loadingwork, the bucket can be raised in a shorter time. Corresponding to theabove case, it is also possible to modify the control process such thatonly the flow control valves 15, 66 for the boom hydraulic cylinders 5a, 5 b are opened in S24 where the three operations signals (1)(3)(5)are produced.

While the above embodiment uses solenoid proportional valves withpressure compensating functions as the flow control valves 15-20, 65-70and the bypass valve 21, the present invention is not limited to such anarrangement. The use of solenoid proportional valves with pressurecompensating functions is preferable from the standpoint of ensuringgood operability because the hydraulic fluid can be always distributedat predetermined flow rates regardless of fluctuations in load of thehydraulic cylinders. But if the hydraulic fluid can be distributed tothe hydraulic cylinders at desired flow rates without using pressurecompensating functions in the intended work, solenoid proportionalvalves with no pressure compensating functions may be used case by case.Further, while the above embodiment uses, as the flow control valves15-20, 65-70 and the bypass valve 21, solenoid proportional valveshaving openings varied in proportion to command signals, the solenoidproportional valves may be simple solenoid on/off valves. In this case,the operation of the solenoid valves under proportional control (seeS10, S12, S13, S20, S21, S22 in FIG. 4) is not achieved in theabove-explained embodiment, but the advantage of reducing the pressureloss caused by hoses, steel pipes or the likes which constitute thelines, as compared with the hydraulic driving system employing theconventional structure can also be provided through the simple on/offoperation. Further, switching valves of the hydraulic pilot operatedtype may be used instead of the solenoid valves. In this case, althoughthere may occur a lag in switching time among the control valves 10 a-f,switching valves 15-20, 65-70 and the bypass valve 21, a necessaryresponse level can be achieved by increasing the diameter of pilot linesor raising the value of a pilot pressure.

While the above embodiment has been explained as constituting each ofthe main lines 105-107, 115-117, the branch lines 150A-F and the supplyline 100 by two or three hoses (or steel pipes, etc.), it is apparentthat those lines may be each formed of one hose (or steel pipe, etc.) ifthere are no restrictions, mentioned above, upon the diameter ofhigh-pressure hoses available in the market.

Moreover, the flow control valves 15-20, 65-70 may be constructed ofseat valves which generate a smaller pressure loss than the controlvalves 10. An example of the construction in such a case will bedescribed below with reference to FIGS. 7 and 8. FIG. 7 is a detailedview showing one 16 of the above flow control valves, by way of example,extracted from FIG. 1, and FIG. 8 is a view showing the structure of aseat valve corresponding to the construction of FIG. 7. Since thepressure compensating functions are not necessarily required in the flowcontrol valves 15-20, 65-70 as stated above, the following descriptionwill be made of an example of the case having no pressure compensatingfunctions.

In FIG. 8, a seat valve 203 fitted to a casing 202 includes a seatportion 203A for communicating/cutting off between an inlet line 221communicating with the main line 105 and an outlet line 231 connected tothe branch portion 151A through a check valve, an end surface 203C forbearing the pressure in the outlet line 231, an end surface 203Bpositioned on the opposite side to the end surface 203C for bearing thepressure in a back pressure chamber 204 formed between itself and thecasing 202, and a throttle slit 203D for communicating between the inletline 221 and the back pressure chamber 204. Also, a pilot line 205 forcommunicating the back pressure chamber 204 and the outlet line 231 isformed in the casing 202, and a variable throttle portion 206constructed of a proportional solenoid valve and adjusting a flow ratethrough the pilot line 205 in response to a command signal 201 isdisposed midway the pilot line 205.

In the above construction, the pressure in the inlet line 221 isintroduced to the back pressure chamber 204 through the throttle slit203D, and the seat valve 203 is pressed downward in the drawing by theintroduced pressure so that the seat portion 203A cuts off between theinlet line 221 and the outlet line 231. When the desired command signal201 is applied to open the variable throttle portion 206, the fluid inthe inlet line 221 flows out to the outlet line 231 through the throttleslit 203D, the back pressure chamber 204, the variable throttle portion206 and the pilot line 205. This flow lowers the pressure in the backpressure chamber 204 as a result of the throttling effect produced bythe throttle slit 203D and the variable throttle portion 206.Accordingly, the force acting upon the end surface 203A, the end surface203C and an end surface 203E becomes greater than the force acting uponthe end surface 203B, whereupon the seat valve 203 is moved upward inthe drawing, causing the fluid in the inlet line 221 to flows outdirectly to the outlet line 231. At this time, if the seat valve 203 isexcessively raised, the throttling opening of the throttle slit 203D isincreased to raise the pressure in the back pressure chamber 204,thereby moving the seat valve 203 downward in the drawing.

In this way, since the seat valve 203 is stopped at an appropriateposition where the throttling degree of the throttle slit 203D isincreased corresponding to the throttling degree of the variablethrottle portion 206, a desired flow rate of the fluid passing from theinlet line 221 to the outlet line 231 can be controlled in accordancewith the command signal 201.

Note that the above embodiment has been explained as applying thepresent invention to a hydraulic excavator of the backhoe type, but thepresent invention is also applicable a variety of construction machinesincluding swing bases and front devices other than the backhoe type.

According to the present invention, the number of supply/return lines isreduced in most areas of the front device as compared with the case ofemploying the conventional structure. Correspondingly, the total lengthof hoses, steel pipes or the likes can be shortened as a whole of thehydraulic excavator and a pressure loss in the entirely of the hydrauliccircuit can be reduced. It is therefore possible to lessen the energyloss, increase the operating speeds of the hydraulic cylinders, andimprove the working efficiency. Also, when all the first flow controlmeans are in the neutral positions, the hydraulic fluid from the otherhydraulic pump is all returned to the hydraulic reservoir through thethird flow control means. This arrangement allows the third flow controlmeans to be disposed midway of the shortest distance between the otherpump and the hydraulic reservoir. The loss caused in the neutralcondition can therefore be minimized to a lower level than caused in thecase employing the conventional structure.

What is claimed is:
 1. A hydraulic drive system for hydraulic excavatorsequipped on a hydraulic excavator comprising an excavator body and afront device made up of a plurality of front members coupled to saidbody to be rotatable in the vertical direction, said front memberincluding a boom, an arm and a bucket, said hydraulic drive systemcomprising a hydraulic reservoir provided on said body, at least onehydraulic pump, a plurality of hydraulic cylinders, including a boomcylinder, an arm cylinder, and a bucket cylinder for respectivelydriving said boom, arm and bucket, a plurality ofpressure-uncompensated-type flow control valves provided on said bodyfor respectively introducing a hydraulic fluid delivered from saidhydraulic pump to said plurality of hydraulic cylinders by a flow ratevariable according to a load pressure and controlling operation of thecorresponding hydraulic cylinders, and a plurality of first connectinglines provided on said front device for respectively connecting saidflow control valves and ones of the bottom and rod sides of thecorresponding hydraulic cylinders, wherein: said hydraulic drive systemfurther comprises at least one other hydraulic pump provided on saidworking machine body separately from said hydraulic pump, a deliveryline to which is introduced a hydraulic fluid delivered from said otherhydraulic pump and a reservoir line for introducing the hydraulic fluidto said hydraulic reservoir, said delivery line and said reservoir linebeing both provided on said body, a second connecting line provided onsaid front device and connected at one side thereof to said deliveryline and extended so that the other side end portion thereof ispositioned at least near said bucket cylinder and at least a partthereof is shared in common as to a supply of a hydraulic fluid to saidboom cylinder, arm cylinder, and bucket cylinder, a plurality of firstlines provided on said front device for forming another hydraulic fluidsupplying route not passing said flow control valves and each having oneside connected respectively to said second connecting line so as to bebranched therefrom, the other side of each of said first lines on theopposite side to said one side connected respectively to at least thoseof said plurality of first connecting lines which are connected to thebottom sides of said hydraulic cylinders, a plurality of first flowcontrol means provided respectively in said plurality of first lines forallowing the hydraulic fluid to flow from said other hydraulic pumptoward said hydraulic cylinders through variable throttles which controlrespective flows of the hydraulic fluid to desired throttled flow rates,but cutting off flows of the hydraulic fluid from said hydraulic pump, athird connecting line provided on said front device and connected at oneside thereof to said reservoir line and extended so that the other sideend portion thereof is positioned at least near said bucket cylinder andat least a part thereof is shared in common as to a discharge of ahydraulic fluid from said boom cylinder, arm cylinder, and bucketcylinder, a plurality of second lines provided on said front device forforming another hydraulic fluid discharging route not passing said flowcontrol valves and each having one end branched from and connected tosaid third connecting line, the other end of each of said second lineson the opposite side to said one end connected to said third connectingline being connected respectively to at least those of said plurality offirst connecting lines which are connected to the bottom sides of saidhydraulic cylinders, a plurality of second flow control means providedrespectively in said plurality of second lines for allowing thehydraulic fluid to flow from said hydraulic cylinders toward said thirdconnecting line through variable throttles which control respectiveflows of the hydraulic fluid to desired throttle flow rates, but cuttingoff flows of the hydraulic fluid from said third connecting line towardsaid hydraulic cylinders, and third flow control means provided in aline branched from said delivery line within said working machine bodyfor supplying the hydraulic fluid delivered from said other hydraulicpump to said first lines at a desired flow rate and returning theremaining hydraulic fluid to said hydraulic reservoir, said plurality offirst lines including a first line for said boom connected to a part ofsaid other side of said second connecting line near said boom cylinderso as to be branched therefrom, a first line for said arm connected to apart of said other side of said second connecting line near said armcylinder so as to be branched therefrom, and a first line for saidbucket connected to a part of said other side of said second connectingline near said bucket cylinder so as to be branched therefrom, saidplurality of second lines including a second line for said boomconnected to a part of said other side of said third connecting linenear said boom cylinder so as to be branched therefrom, a second linefor said arm connected to a part of said other side of said thirdconnecting line near said arm cylinder so as to be branched therefrom,and a second line for said bucket connected to a part of said other sideof said third connecting line near said bucket cylinder so as to bebranched therefrom, said plurality of first flow control means includinga first flow control means for said boom provided at a part of saidfirst line for said boom near said boom cylinder, a first flow controlmeans for said arm provided at a part of said first line for said armnear said arm cylinder, and a first flow control means for said bucketprovided at a part of said first line for said bucket near said bucketcylinder, said plurality of second flow control means including a secondflow control means for said boom provided at a part of said second linef o r said boom near said boom cylinder, a second flow control means forsaid arm provided at a part of said second line for said arm near saidarm cylinder, and a second flow control means for said bucket providedat a part of said second line for said bucket near said bucket cylinder.2. The hydraulic drive system for hydraulic excavators according toclaim 1, wherein the other side of at least one of said first line forsaid boom, said first line for said arm, and said first line for saidbucket on the opposite side to said one side connected to said secondconnecting line is connected to that of said plurality of firstconnecting lines which is connected to the rod side of said hydrauliccylinder, and said first flow control means provided in said at leastone first line allows t he hydraulic fluid to flow from said otherhydraulic pump toward the rod side of said hydraulic cylinder through avariable throttle for controlling a flow of the hydraulic fluid to adesired throttled flow rate, but cuts off a flow of the hydraulic fluidfrom the rod side of said hydraulic cylinder toward said other hydraulicpump.
 3. The hydraulic drive system for hydraulic excavators accordingto claim 1, wherein the other side of at least one of said first linefor said boom, said first line for said arm, and said first line forsaid bucket on the opposite side to said one side connected to saidsecond connecting line is connected to that of said plurality of firstconnecting lines which is connected to the rod side of said hydrauliccylinder, said first flow control means provided in said at least onefirst line allows the hydraulic fluid to flow from said other hydraulicpump toward the rod side of said hydraulic cylinder through a variablethrottle for controlling a flow of the hydraulic pump, the other side ofat least one of said second line for said boom, said second line forsaid arm, and said second line for said bucket on the opposite side tosaid one side connected to said third connecting line is connected tothat of said plurality of first connecting lines to which said at lestone first line is connected and which is connected to the rod side ofsaid hydraulic cylinder, and said second flow control means provided insaid at least one second line allows the hydraulic fluid to flow fromthe rod side of said hydraulic cylinder toward said hydraulic reservoirthrough a variable throttle for controlling a flow of the hydraulicfluid to a desired throttled flow rate, but cuts off a flow of thehydraulic fluid from said hydraulic reservoir toward the rod side ofsaid hydraulic cylinder.
 4. The hydraulic drive system for excavatorsaccording to claim 1, further comprising control means for controllingsaid plurality of flow control valves and said first flow control meansto be driven in a correlated manner so that just before or after thehydraulic fluid through at least one of said plurality of flow controlvalves is fully supplied to the corresponding first line, the hydraulicfluid through the corresponding first control means starts to besupplied to the corresponding first connecting line.
 5. The hydraulicdrive system for hydraulic excavators according to claim 2, furthercomprising control means for driving said first flow control meansdisposed in at least one of said plurality of first lines which isconnected to the rod side of said hydraulic cylinder, thereby supplyingthe hydraulic fluid from said other hydraulic pump to the rod side ofsaid hydraulic cylinder, and at the same time driving said second flowcontrol means disposed in the second line which is connected to thebottom side of the corresponding hydraulic cylinder, thereby drainingthe return hydraulic fluid from the bottom side of the correspondinghydraulic cylinder to said hydraulic reservoir.
 6. The hydraulic drivesystem for hydraulic excavators according to claim 1, further comprisinga plurality of operating means which output operation signals forcontrolling respective stroke amounts of said plurality of flow controlvalves and control means for receiving said output operation signalsfrom said operating means and controlling said flow control valves andsaid first flow control means to be driven in a correlated manner, saidcontrol means operating in a manner such that in a first input amountarea where input amounts of said operating means are relatively small,said flow control valves are moved over strokes at a relatively smallratio with respect to an increase of the input amounts of said operatingmeans, thereby supplying the hydraulic fluid to the corresponding firstconnecting lines, and that in a second input amount area where the inputamounts of said operating means are relatively large, said flow controlvalves are moved over strokes at a relatively large ratio with respectto an increase of the input amounts of said operating means to controlthe flow rate of said flow control valves, thereby supplying thehydraulic fluid to the corresponding first connecting lines, and saidfirst flow control means are moved over strokes at a predetermined ratiowith respect to an increase of the input amounts of said operatingmeans, thereby supplying the hydraulic fluid to the corresponding firstconnecting lines through the corresponding first lines.